What's an Oak Gall?

Oak apple or oak gall is the common name for a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from 2 to 4 centimetres in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae.

What is inside an oak gall?

“Oak apple galls” are leaves that have developed into a thin sphere because wasps have laid eggs inside of the leaf. Inside the gall is a tiny wasp larva. Most galls, especially on leaves, do not hurt the oak tree, and the wasps aren't harmful to people either. ... Here, wasp larvae hatch and feed on roots of the oak tree.

What is the purpose of an oak gall?

Oak galls, also called oak apples, are a common phenomenon produced by the oak trees' reaction to wasps that lay their eggs inside of the oak bark. Oak galls are high in tannic acid and have been used traditionally as medicine by many cultures around the world; they are also a source of dye and tanning material.